The present invention is directed to diffractive optical structures and, more particularly, to diffractive optical structures exhibiting significant polarizing selectivity, particularly those employing a subwavelength optical grating structure. Diffractive optical structures exhibiting significant polarizing selectivity according to the invention may be advantageously used for beam splitting.
Polarizing beam splitters, such as those formed from a multi-layer dielectric stack, are known for selecting light of a particular polarization. However, such conventional beam splitters are relatively difficult to manufacture and design. Further, such stacked dielectric polarizing splitters are not particularly easy to integrate with other optical components.
Although diffractive polarization is a known problem in spectroscopy, the use of diffractives in beam separation, splitting or routing has not been previously recognized. Instead, the polarization selectivity exhibited by a diffractive was considered an undesired problem to be corrected.
The difference in the transmission of transverse electric mode (TE) and transverse magnetic mode (TM) polarized light in diffractive structures has also been documented in the antireflection context. However, the objective of the research on such diffractive subwavelength structures has been to minimize the difference in transmission in TE and TM light for near normal incident light. Such structures which equally transmit normally incident TE and TM polarized light are of little use when the selection of only one polarization is sought.
Applicants of the present application have discovered that diffractive optics may be used to separate a incident beam, acting as a polarizing beam splitter. Although these characteristics are most apparent in subwavelength structures, diffraction gratings having periods greater than the wavelength of the illuminating beam can also display significant polarization selectivity. Because the polarizing selectivity is best exhibited by subwavelength structures, the present application will discuss the inventive principles disclosed herein primarily in terms of such subwavelength structures. However, the principles disclosed herein also apply to diffractives employing grating periods greater than the wavelength of the illuminating beam so long as significant polarizing selectivity is exhibited.
Diffractive structures may have periodically repeating features, either over the entire structure or with different periods over different portions of the structure. Such structures also may have more than one period spatially superimposed. The period of a uniformly periodic diffraction grating may be defined as the smallest distance over which the grating structure repeats.
If the grating period of the subwavelength structure is less than half of the incident beam""s wavelength, then only the zeroth diffraction order propagates, and all other diffraction orders are evanescent. Subwavelength structures show interesting properties such as antireflection, form birefringence, and emulation of distributed index materials.
An object of the invention is to provide a polarizing beam splitter which substantially obviates one or more problems or limitations of the conventional layered dielectric beam splitters.
Another object of the present invention is to use a diffractive beam splitter to separate beams.
Still another object of the present application is to develop a beam splitter or router that can be easily integrated in an assembly with other optical components.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.
To achieve the objects and in accordance with the purpose of the invention, as embodied and broadly described herein, there is provided a polarizing beam splitter, including a diffraction grating provided on a surface and arranged at an oblique angle to an incident light beam, the diffraction grating including a plurality of repeating raised portions having a period selected to differentially transmit and reflect differently polarized components of the light beam. According to one preferred embodiment this selective transmission is accomplished with a subwavelength grating period.
Also in accordance with the present invention there is provided a beam router, including a prism having a beam of light incident thereon; and a diffraction grating integrated onto a surface of the prism and operating as a polarizing beam splitter.
Also in accordance with the present invention there is provided a method of splitting an incident light beam in dependence on the direction of polarization thereof including providing a diffraction grating on a substantially planar surface, the diffraction grating having a plurality of repeating raised portions having a period selected to differentially transmit and reflect differently polarized components of the light beam; and orienting the plane of diffraction grating at an angle a to the incident light beam, where a is less than 90% and greater than 0%, the incident light beam having a first polarized component parallel to the plane of the diffraction grating and a second polarized component perpendicular to the first polarized component, wherein the first polarized component is substantially reflected by the diffraction grating while the second polarized component is substantially transmitted.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.
The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments of the invention and, together with the description, serve to explain the principles of the invention.